1. Field of the Invention
This invention relates to free electron lasers (FEL), and more particularly to high power microwave and millimeter wave FELs.
2. Description of the Related Art
An FEL amplifies short-wavelength radiation by stimulated emission, using a beam of relativistic electrons. The electrons are not truly "free", since they are under the influence of magnetic forces which cause them to radiate, but they are "free" in the sense that they are not bound into atoms as in the case of a conventional laser. The FEL radiation is usually produced by passing the electrons down a magnetic device known as an undulator or "wiggler", in which the electrons are forced to execute a periodic oscillatory trajectory in space. The wiggler may produce a helical field using a bifilar helical winding, a linearly polarized field using a set of alternating polarity magnets, an electrostatic field, or an electromagnetic field. In the presence of a wiggler field and the electromagnetic wave being amplified, the traveling electrons see an oscillating field everywhere except in certain regions where they bunch together and travel in synchronism. This bunching enables them to radiate coherently as they are oscillated by the wiggler field, and release appreciable amounts of power. A background article on this type of device is A. Hasegawa, "Free Electron Laser", Bell Technical Journal, vol. 57, no. 8, Oct. 1978, pgs. 3069-3089.
In any event, the electron density of the electron beam (E-beam) is very high, on the order of 10.sup.9 to 10.sup.12 electrons per cubic centimeter. The electron beam typically travels through a vacuum or a near vacuum.
In a typical FEL, the electrons are accelerated in a diode structure or electron gun ("E-gun"). The gun typically operates in pulsed form, from single-shot to 1000 Hz. A typical gun consists of a hot or cold cathode and either focusing elements or a guide magnetic field. The acceleration of the electrons in the electron beam ("E-beam") primarily occurs in the diode structure or E-gun and in any acceleration stages located past the E-gun. The energy of the electron beam produced by the E-gun and accelerator may be in the relativistic range for electrons, that is, on the order of mc.sup.2 =510 kV. The energy may be as high as 1000 MV. The typical operating range for a microwave FEL could be about 150 to 700 KV.
Most FEL applications are for the generation of radiation in the ultraviolet to infrared bands and require a large, high energy accelerator. However, FELs having High Power Microwave (HPM) outputs are being developed for certain applications. Some of these applications are discussed in Florig, H. Keith, "The Future Battlefield: A Blast of Gigawatts?", IEEE Spectrum, March, 1988, pages 50-54.
FIG. 1 depicts a typical prior art HPM FEL. In FIG. 1, FEL 10 has an electron gun having an anode 12 and a cathode 14. The gun emits an accelerated electron beam 16 that is focused and confined by a guidefield solenoid 18, and passed through a reflector 20, and a metal waveguide 24. The beam also passes through a wiggler magnet array 22 which imposes an oscillating trajectory upon the electron beam, causing the E-beam to emit radiation. The E-beam and the emitted radiation 30 pass through FEL output 26. The E-beam is collected and recycled by a depressed collector 28, while the output radiation 30 proceeds in a linear path towards a target. Assuming that the combined length of the wiggler and reflector/resonator assembly is 50 centimeters and that a 225 kV E-beam is used, the FEL depicted in FIG. 1 will generate about 60 kW of power at a frequency of about 30 GHz.
FIG. 2 is a graph depicting typical prior art FEL operating conditions. FIG. 2 shows the frequency and gain of an FEL as a function of the operating beam voltage.
The FEL depicted in FIG. 1 uses a single high-current density E-beam. This approach has several disadvantages. First, field-emission diodes are typically required as electron guns to generate the high-current E-beams of the type needed for a HPM FEL. However, such field-emission diodes have very short closure times, on the order of nanoseconds, and therefore limited pulse lengths and limited repetition times. Field-emission diodes also tend to produce poor quality electron beams, thereby reducing the portion of the beam involved in the interaction as well as its conversion efficiency.
Second, these single high-current density E-beams have a tendency to "blow-up" due to their own very high space charge, and thus require large guiding magnetic fields for beam propagation.
A third problem with single high-current density E-beams are the power-handling difficulties created at the output end of the FEL.
A single gun, sheet or ribbon electron beam FEL is disclosed by Booske, J. H., in "Low-Voltage, Megawatt Free-Electron Lasers At a Frequency Near 300 GHZ", Microwave and Particle Beam Sources and Propagation (1988), SPIE Volume 873, pages 133-142. The Booske article discloses a millimeter-wave FEL operating at a frequency between 150 and 300 GHz, with an output of about 1 megawatt. This device uses a thermionic Pierce gun as the accelerator. The drawbacks of using such thermionic guns include limited current density, the requirement of heater power, radiation of heat, and susceptibility to poisoning. Also, grid control in such guns is difficult since the grid must operate at the high voltage of the gun cathode.